This is a single-institution Phase I study. The objectives of this trial are: 1) to characterize the safety profile and determine the dose- limiting toxicity (DLT) and maximum tolerated dose (MTD) of temozolomide when given orally, twice a day (q 12 hours) over 5 consecutive days, after a bolus dose of 200 mg/M2, to adult patients with advanced cancer; 2) to characterize the pharmacokinetics of temozolomide when given twice a day (q 12 hours) over 5 days, after a bolus dose of 200 mg/M2, to adult patients with advanced cancer; 3) to evaluate the biochemical modulary effect of temozolomide on the depletion of alkyltransferase (ATase) by measuring ATase levels in circulating peripheral blood lymphocytes and, at the MTD, in tumor biopsies; and 4) to evaluate the antitumor activity of temozolomide when given twice a day (q 12 hours) over 5 days, after a bolus dose of 200 mg/M2, to adult patients with advanced cancer. Currently patients are being treated at a total dose level of 1010 mg/M2 (range = 650 mg/M2 to 1325 mg/M2). This study will continue to accrue patients.